A configuration of a self-excited AVR synchronous generator including a generation winding 2 and an excitation winding 3 wound on a stator side of the generator 1, a field winding 6 wound around a rotor 5 to be rotated by a drive source (engine) 4, a permanent magnet 7 fitted to the rotor 5 for generating an excitation current, and an automatic voltage regulator (AVR) 10 which regulates a current to be supplied to the field winding 6 as shown in FIG. 4, is described in Patent Literature 1.
The automatic voltage regulator (AVR) 10 connected to the field winding 6 via a brush 8 includes a commutator 11 having an input side to which both ends of the excitation winding 3 are connected, a capacitor 12 provided between the commutator 11 and the ground to smooth an output voltage of the commutator 11, a flywheel diode 13 connected in parallel to the field winding 6, a transistor 14 which is controlled to be turned ON/OFF to supply a field current to the field winding 6, and a field current drive circuit (field current driving means) 15 which PWM-controls a field current. One end of the field winding 6 is connected to the output side of the commutator 11, and the other end of the field winding 6 is connected to the collector side of the transistor 14.
The flywheel diode 13 is provided for absorbing a surge voltage caused in case of power supply stop and smoothing the field current when PWM-controlling the field current flowing in the field winding 6.
The output side of the generation winding 2 is connected to a load 9 via a brush 8, and is configured so that a detected output voltage is input into the field current drive circuit 15.
The automatic voltage regulator (AVR) 10 operates to hold a voltage to be output from the generation winding 2 at a voltage set in advance by regulating a current to be supplied to the field winding 6 by turning ON/OFF the transistor 14.
In the self-excited AVR synchronous generator, when a capacitive load which is a condensive load is connected as the load 9, magnetization of the rotor 5 occurs due to armature reaction. Therefore, due to a predetermined or more condensive load current, as shown in FIG. 5B, a phenomenon occurs in which a back electromotive voltage is generated in the field winding 6 of the rotor 5. At this time, the back electromotive voltage (overvoltage) is applied to the AVR as a rotor excitation control unit, so that when it has no protective function, a commutating device, etc., such as the capacitor 12 inside the AVR 10 may be broken by the overvoltage. FIG. 5A is a schematic circuit diagram showing a current flowing in the excitation winding 3 and the field winding 6 in a normal state where no condensive load is connected. In FIGS. 5A and 5B, instead of the transistor 14 as a switching element in FIG. 4, a field current control FET 14 is used.
Conventionally, as a condensive load protecting short circuit for suppressing a back electromotive voltage, as shown in FIG. 6, a self-bias circuit 40 including two bipolar transistors Darlington-connected is used. With this circuit, due to a back electromotive force according to armature reaction of a condensive load, a base current ib flows from the rotor (field winding 6) to the transistor 42 via a lead resistor 41, and a short-circuit current is flows according to short-circuiting of the transistor 43.